I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for transmitting packetized voice and data over communication networks.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard, or a CDMA system that conforms to the “TIA/EIA/IS-2000-2 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” hereinafter referred to as the cdma2000 standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention and incorporated by reference herein.
In this specification, base station refers to the hardware with which the remote stations communicate. Cell refers to the hardware or the geographic coverage area, depending on the context in which the term is used. A sector is a partition of a cell. Because a sector of a CDMA system has the attributes of a cell, the teachings described in terms of cells are readily extended to sectors.
In the CDMA system, communications between users are conducted through one or more base stations. A first user on one remote station communicates to a second user on a second remote station by transmitting data on the reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on the forward link of the same base station, or a second base station, to the second remote station. The forward link refers to transmission from the base station to a remote station and the reverse link refers to transmission from the remote station to a base station. In IS-95 and cdma2000 systems, the forward link and the reverse link are allocated separate frequencies.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 and cdma2000 standards are capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of the present invention and incorporated by reference herein.
A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques, which require significantly larger delays than those that can be tolerated by voice services, can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. Pat. No. 5,933,462, entitled “SOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDS,” filed Nov. 6, 1996, assigned to the assignee of the present invention and incorporated by reference herein.
Another significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal transmission rate for all users and a maximum tolerable value for the error rates of the speech frames. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system is typically defined as the total delay incurred in the transfer of a predetermined amount of data, hereinafter referred to as a data packet.
Yet another significant difference between voice services and data services is that the former requires a reliable communication link which, in the exemplary CDMA communication system, is provided by soft handoff. Soft handoff results in redundant transmissions from two or more base stations to improve reliability. However, this additional reliability is not required for data transmission because the data packets received in error can be retransmitted. For data services, the transmit power used to support soft handoff can be more efficiently used for transmitting additional data.
Various protocols exist for transmitting packetized data over communication systems so that information arrives at its intended destination. One such protocol is “The Internet Protocol,” RFC 791 (September, 1981). The internet protocol (IP) breaks up data messages into packets, routes the packets from a sender to a destination, and reassembles the packets into the original data messages at the destination. The IP protocol requires that each data packet begins with an IP header containing source and destination address fields that uniquely identifies host and destination computers. The transmission control protocol (TCP), promulgated in RFC 793 (September, 1981), is responsible for the reliable, in-order delivery of data from one application to another.
A typical TCP/IP header is 40 bytes long, wherein 20 bytes are required to satisfy the IP protocol and 20 bytes are required to satisfy the TCP protocol.
In a slow communication link, the overhead required for transmitting TCP/IP headers may be unacceptable for end users. As is well known in the art, this header overhead problem has been solved with compression techniques, such as the one promulgated by RFC 1144 (February, 1990), entitled “Compressing TCP/IP Headers for Low-Speed Serial Links,” wherein a data packet undergoes differential coding. Compression is accomplished with a compressor that receives a header and extracts only those fields in the header that differ from the fields in the previous header. If the differences in the changing fields are sent rather than the fields themselves, a significant savings can be achieved. Consequently, a decompressor at the receiving end must be synchronized with the compressor so that the proper ordering of compressed headers is maintained. If the compressor and the decompressor are not in the same state, then the decompressor must be resynchronized with a transmission of the first, uncompressed packet of a compressed packet sequence.
Header compression is often used in wireless communication systems to improve the bandwidth and power efficiency of the link by increasing the percentage of the link used for the information payload. Unfortunately, due to the nature of wireless communication systems, temporary interruptions in the delivery of information packets are not uncommon. The occurrence of any such interruption may cause significant delays due to the need to retransmit a resynchronization packet to resynchronize a decompressor at a target locale and a need to re-negotiate traffic parameters between the compressor end and the decompressor end. There is a present need to reduce the amount of delay caused by the transmission of resynchronization and re-negotiation information, and to increase the data throughput rate of the system accordingly. The need to increase the data throughput rate is always present in modern communication systems that support a variety of applications.